The tumor suppressor CYLD acts as a deubiquitinase for mTOR to constrain its activity

Area of Science:

• Cellular signaling pathways
• Molecular biology
• Ubiquitin-modified proteins

Background:

• Mechanistic/mammalian target of rapamycin (mTOR) signaling is crucial for cellular functions, health, and disease.
• The role of mTOR ubiquitination in regulating its activity is not well understood.
• CYLD is a known tumor suppressor, but its direct targets and functions in mTOR signaling are unclear.

Purpose of the Study:

• To identify novel regulators of mTOR signaling through unbiased screening.
• To elucidate the role of CYLD in controlling mTOR activity and ubiquitination.
• To investigate the link between CYLD, mTOR hyperactivation, and CYLD cutaneous syndrome (CCS).

Main Methods:

• Unbiased RNA interference (RNAi) screen to identify mTOR regulators.
• Biochemical assays to study protein interactions and deubiquitinase activity.
• Cellular assays to assess mTORC1/2 activity, protein synthesis, and cell death.
• Analysis of *C. elegans* models and human patient biopsies.

Main Results:

• CYLD deubiquitinase (DUB) was identified as a direct negative regulator of mTORC1 and mTORC2.
• CYLD removes K63-linked ubiquitin chains from mTOR, controlling its activity.
• Loss of CYLD function leads to mTORC1/2 hyperactivation, increased protein synthesis, cell growth, and resistance to cell death.
• Silencing of the *C. elegans* ortholog *cyld-1* reverses lifespan extension in low-TORC1 mutants.
• CYLD inactivation correlates with mTORC1 hyperactivation in skin biopsies from CCS patients.

Conclusions:

• CYLD acts as a critical sentinel of mTOR hyperactivation through direct control of mTOR ubiquitination.
• Dysregulated mTOR activity due to CYLD inactivation may contribute to the pathogenesis of CCS tumors.
• Targeting the CYLD-mTOR axis could offer therapeutic strategies for CCS and related conditions.